1. Field of the Invention
The present invention relates to a CCD storage module having storage positions which are arranged in cascade and which are formed with the aid of electrodes arranged in insulated fashion above a semiconductor substrate, and more particularly to such an arrangement wherein, in respect of each storage position, an item of information incoming as a n-digit binary number is stored in that a quantity of charge composed of i unit charges is stored in respect of each storage position, where i corresponds to the value of the binary number of the information.
2. Description of the Prior Art
Charge coupled device stores (CCD stores) or charge shift arrangements are well known in the art. In this respect, one may refer to the publications "Electronics", June 21, 1971, pages 58ff and to the "Bell System Technical Journal", Vol. 940, Edition 4/1970, pages 587- 593. These arrangements fundamentally consist of a semiconductor substrate, an electrically insulating layer carried on the substrate, and electrodes arranged on the insulating layer. The electrodes are electrically isolated from one another. The principle of the CCD store is based on connecting suitably selected voltages to the electrodes in order to achieve favorable potential conditions at the boundary surface between the insulating layer and the semiconductor substrate, where minority charge carriers can be stored. At a suitable instant time, a special write-in device writes minority charge carriers into these potential wells or blocks the inflow of minority charge carriers to the potential wells, as governed by the particular item of information to be written in. Therefore, charge shift arrangements of this kind are particularly suitable for shift registers.
It is also well known in the art to employ charge shift arrangements as memories. For this purpose, at the beginning of the charge shift arrangement constructed as a shift register, which in the following will be referred to as a "storage field", there is provided an input stage, at the output of the storage field there is provided an output stage, and between the output stage and the input stage there is provided a regenerator stage. This type of construction can be referred to as a storage loop. As the individual storage positions of a storage loop of this type can store the information only for a specific length of time, the stored information continuously circulates. In so doing, the information is also conducted across the regenerator stage and thus regenerated during each cycle.
A goal in the development of CCD stores is to achieve the greatest possible storage density on a semiconductor substrate. Several methods of achieving this goal have already been proposed.
A first method of achieving the greatest possible storage density is a reduction in structure. This method is the primary object of the technological development.
A second method resides in improved circuitry measures. The E/B principle which is described, for example, in the "IEEE Journal of Solid State Circuits", February 1976, Vol. SC-11, No. 1, Section C, pages 8-9, and which is applied to a storage field arranged in accordance with the SPS principle is along these lines.
A further method is likewise described in the above-mentioned publication, in which it is stated that it should also be possible to store more than one bit in respect of each storage position of a CCD store. This principle is referred to as multilevel storage (MLS). With this principle, the property of CCD arrangements of being able to process analog signals is employed. In order to be able to store, for example, a four-digit binary number, 16 different charge levels are required. In accordance with the binary signal present in parallel on, for example, four lines (in accordance with the four bits of the binary number), numbers 0, 1, 2 ... 14 or 15 times a suitably selected charge unit, which in the following will be referred to as a "unit charge", is input into the CCD storage field. This necessitates a decoder circuit of appropriate construction. An evaluator circuit at the output of the CCD storage field must establish the size of the incoming quantity of charge and reconvert this 1-out-of-16 signal into the four-bit signal. The information is then returned, in this form, to the input of the storage field, thus to the decoder circuit.
Therefore, if a n-digit binary number is to be stored beneath a storage position of a CCD storage module, it is necessary to transmit a quantity of charge composed of i unit charges to the storage position, where i is the value of the binary number of the information, and i.ltoreq. (2.sup.n -1).